When retrieving pending meter share requests using GET /share/meter/pending/list, the response will always provide the attribute <meterID>, which is a unique ID assigned to each meter created in Portfolio Manager.
The example response provided at here also shows an example custom field called "Meter Number". Organizations exchanging data via web services are able to set up custom attributes for objects in Portfolio Manager. Often, this is done to track attributes like utility account numbers, meter names, etc. In the example provided, the "Meter Number" custom field might refer to the utility meter number and would have been set up in advance by the provider. Data in the field would have been provided by the end user during the sharing process.